Dislocation-particle interactions in magnesium alloys

The aim of this work is to investigate the fundamentals of dislocation-particle interactions in a Mg-1%Mn-1%Nd (wt.%) (MN11) alloy in order to better understand the limited precipitation strengthening exhibited by this material. A combined approach including micromechanical testing, slip trace analysis, and high resolution transmission electron microscopy (TEM) is put in place to analyze the interaction between basal and non-basal dislocations with the Mg-Nd plate precipitates that are characteristic of the investigated alloy. It is shown that basal dislocations can easily shear the precipitates, both when their long axis lays parallel and perpendicular to the matrix c axis. This is attributed to the high coherency between the matrix and the particles, which is characterized in detail. It is additionally shown that pyramidal 〈c + a〉 dislocations, on the contrary, interact with precipitates both in the form of shearing along the precipitate pyramidal planes, as well as by Orowan looping. This study reveals that pyramidal dislocations are able to shear the plate precipitates along pyramidal planes when there is a good alignment between the slip plane of the incoming matrix dislocation and the outgoing precipitate pyramidal plane. Otherwise pyramidal dislocations sliding on matrix planes with small geometric compatibility with the precipitate pyramidal planes bypass these particles by an Orowan looping mechanism.